In many applications it is highly desirable to maintain the speed of a motor driven by direct electrical current at a constant value with a high degree of precision. The precise control of motor speed typically dictates the use of a servo feedback control loop for regulating motor speed. That is, a signal indicative of actual motor speed is derived from the turning shaft of the motor and is combined with a reference speed signal to command the motor to move at a precise rate. Thus, a means of compensation is provided for drift in amplifying signals which occur in electrical components. Such drifting of amplifying signals may be caused by temperature changes, component aging, or humidity. The use of a servo feedback control allows the motor speed to be held to a commanded value in a system which responds immediately to minor deviations from this value.
Synchronous motors are used in applications where highly precise control of speed is necessary. In synchronous motors, the rotor of the motor is designed to "lock" at a synchronous speed and maintain that speed under all normal loading conditions. Hysteresis type synchronous motors maintain their speed within precise limits, but are quite expensive and are sensitive to voltage changes.
Hysteresis types sychronous motors exhibit a mode of oscillation called hunting. Hunting usually occurs in the frequency range from 1 to 4 hertz and remains as a lingering effect on motor operation some time after the establishment of a change in commanded motor speed. The degree of error in speed of motor rotation that results from hunting produces results that are unacceptable in many devices. For example, in laser beam printing machines in which a laser beam is directed at a rotating polygonal mirror to effectuate a reproduction of printed or illustrated material, rotation of the polygonal mirror at an absolutely constant speed is critical. However, in existing systems the motors utilized to rotate the polygonal mirror are subject to the effects of hunting with the result that copy quality produced from these printers is degraded well below an acceptable level whenever motor load is disturbed. The degradation of quality manifests itself as a wavy print on the copy at the hunting frequency. That is, a line which should be reproduced as a straight line is reproduced having a wavy configuration. The inducement of hunting of a hysteresis synchronous motor may result from one of a number of causes, but frequently occurs as a result of initially bringing the motor up to speed or changing motor speed.
Accordingly, it is a principal object of the present invention to provide a servo control system for a direct current motor that is insensitive to those influences that lead to hunting in conventional synchronous motors. This is achieved by providing the motor with dual control loops. One of the control loops renders the system extremely sensitive to variations in actual motor speed, so that the speed of the motor is not allowed to drift. The other control loop renders the motor insensitive to temporary fluctuations of lower frequency in motor loading.
A further object of the invention is to provide a servo control system for a synchronous motor in which the reference speed signal is provided from a 60 hertz alternating current power line. This allows the synchronous motor to be referenced to a common line frequency with other components of an equipment system, such as other motors. Thus, changes which occur in the 60 hertz line frequency effect all of the system components, uniformly, thus minimizing resulting adverse effects.
A further object of the invention is to provide a servo control system utilizing a plurality of control loops. This decreases the complexity of the servo control system from conventional devices, which typically use some form of phase locking technique to both dampen and regulate the shaft speed of the motor. In such conventional systems the cost of implementing the servo loop increases with increasing complexity. Simplicity is achieved in the present invention, however, by utilizing two loops.